Ferrite transducers



Nov. 27, 1962 H. B. MILLER 3,066,233

PREPARE SLIP OF ELECTROMECHANICALLY ACTIVE CERAMIC ADDWATER,WATER-SOLUBLE GELLING AGENT AND WETTING AGENT AGITATE VIG OROUSLYIN A HEATED CONTAINER TO AERATE POUR INTO PAPER-LINED WIRE BASKETS, COOLTO SOLIDIFY; DRY.

REMOVE PAPER FROM DRY CERAMIC ELEMENTS AND FIRE ELEMENTS TO MATURITYFIG.I

INVENTOR. HAR RY B. MILLER ATTORNEY Nov. 27, 1962 H. B. MILLER FERRITETRANSDUCERS 2 Sheets-Sheet 2 Filed July 25, 1957 FIG;3

FIG.2

FIG.4

INVENTOR. HARRY B. MILLER ATTORNEY the ferroelectric and ferromagneticceramics.

3,ii66,233 Patented Nov. 27, 1952 are 3,066,233 FERRITE TRANSDUCERSHarry B. Miller, Cleveiantl Heights, ()hio, assignor to CleviteCorporation, Cieveland, Ohia, a corporation of Ohio Filed Jul 25, 1951s.N0. 674,2il l 6 Claims. (ill. 310-26) of the transducer with water. Thisapplies also to ultrasonic transducers, such as are used for cleaning,sonic irradiation, etc., which operate in fluid transmission media otherthan water.

; :The impedance matching of transducers to transmission media has longbeen a serious problem in the art. Many of the electromechanicaltransducing materials and elements heretofore available have a fixeddensity and fixed mechanical complianceand, therefore, a fixed acousticcharacteristic impedance, This acoustic characteristic impedance usuallyis much higher than that of the water or other transmission fluidinvolved. This is particularly true of the piezomagnetic metal andceramic materials as well as the ferroelectric ceramics. For example,the normal specific acoustic impedances of ferroelectric lead zirconatetitanate and barium titanate ceramics are in the range from about 20 to30 10 (kg./m. (m./sec.) or rayls as compared to 1.5 1() rayls for water.The unit of specific acoustic impedance used here is the mks. rayl.Nickel has an acoustic impedance of about 42.3)( rayls and ceramicnickel ferrite about 27 10 rayls. Some monocrystalline transducermaterials, e.g., ammonium dihydrogen phosphate (NH.,H PO have relativelylow acoustic impedance and are therefore a comparatively good match towater but transducer elements of these materials suffer from otherdisadvantages: they are limited in size and, therefore, are not suitablefor low frequency resonant operation unless mass-loading is resorted to;they are relatively more expensive to -produce than ceramic elements andare not susceptible of being formed and shaped by ceramic techniques;and, for generating highly directional signals, large heavy arrays ofmonocrystalline elements must be used because of their individual sizelimitations.

Due to the inherent shortcomings of monocrystalline transducer materialsand elements, the trend in recent years has been toward polycrystallinematerials such as Heretofore, the problem of impedance matching thusencountered has beenattacked by resort to various impedancetransformation means such as horns, plates, etc. Such impedance matchingexpedients, however, are undesirable in that they add weight, bulk,complexity and cost to the transducer.

These difficulties and problems are overcome by the present inventionwhich contemplates an electromechanical transducer comprising acellulated body of ferromagnetic ceramic material. Due to the fact thatthe ceramic vmaterial is cellulated, its density is less and itscompliance greater than conventional material; thus it has a lowercharacteristic impedance which is a much better match to Water and mostother common transmission media.

It is a general object of the invention to provide piezoelectric ormagnetostrictive transducers, transducer elements and ferromagneticceramic materials which overcome at least one of the problems of theprior art.

It is another general object of the invention to provide a novelferromagnetic ceramic element of low density which is capable of asubstantial magnetostrictive response.

Still another object of the invention is the provision of a noveltransducer element capable of satisfying the practical requirements forunderwater transducer operation.

A further object of the invention is the provision of novelmagnetostrictive transducer elements of cellulated ceramic materialwhich, because of their porous structure, have a reduced mechanicalcharacteristic impedance, thereby making them capable of an effectiveelectromechanical response over a wider frequency bandwidth.

A further object of the present invention is to provide more readilymachinable dielectric ceramic material adapted 'for use inmagnetostrictive transducers.

A still further object of the present invention is to provide a noveltransducer element of ceramic material which has reduced elasticcross-coupling between the parallel and lateral modes and, therefore,has improved piezomagnetic activity in its parallel mode, as comparedwith prior art elements of this general type.

Further objects and advantages of the invention as well as the specificdetails of construction and mode of operation of the transducer elementand the preferred manner of making it will be apparent from thefollowing description taken in conjunction with the subjoined claims andannexed drawings, in which,

FIGURE 1 is a flow diagram of the preferred process of making cellularceramic transducer materials and elements in accordance with the presentinvention;

FIGURE 2 is a perspective view of a magnetostrictive cellular ceramicelement according to the present invention;

FIGURE 3 is a perspective view of a cellular ceramic element as shown inFIGURE 2, with polarizing and sig nal windings schematically indicated;and

FIGURES 4 and 5 are longitudinal sections through electroacoustictransducers for underwater operation which incorporate ceramicmagnetostrictive elements according to the present invention.

The flow diagram in FIGURE 1 illustrates broadly the steps involved inmaking magnetostrictive elements and material in accordance with theinvention. Thus a slip is prepared of raw materials or precursors of amagnetostrictive ceramic material such as nickel ferrite (NiFe O To thisslip is added water, a gelling agent and a wetting agent. The slip isbeaten wtih a food mixer in a heated container to aerate it and thenpoured into paper-lined molds to cool, solidify and dry. The dryelements are removed from the molds and fired to ceramic maturity.

Before proceeding with an example of the method, it is pointed out thatthe method employed and further examples of its application to variouselectromechanically responsive materials are disclosed in detail andclaimed in US. Letters Patent No. 2,892,107 issued to C. K. Gravley andA. L. W. Williams and assigned to the same assignee as the presentinvention.

One class of ferromagnetic materials suitable for practicing the presentinvention are the polycrystalline ceramic ferrites. As is well known,the ferrites have the type formula MFe O (or MO-Fe O where M is adivalent metal, e.g. nickel, cobalt, zinc. The stoichiometric aoeaass 3mol ratio of the metal oxide to the iron oxide is 1:1. In mixed ferritesthe metal oxide component (MO) is a mixture of two or more divalentmetal oxides as, for example, in nickel cobalt ferrite.

For the sake of example, the present invention will be described usingnickel ferrite (NiFe O as a typical ferromagnetic material.

The nickel ferrite is prepared in powder form in any suitable manner anda ceramic slip made of the powder. One suitable slip was made bydispersing dry ferrite powder with about 1% Marasperse (a dispersingagent which is a sodium salt of ligno-sulfonic acid) and 30% water, allpercentages being on a weight basis. The mixture was rendered alkalineby the addition of ammonium hydroxide (NH OH) solution and wet milledfor several hours. Thereafter, a solution of P.V.A. 7124 (a commerciallyavailable polyvinyl alcohol, used as a binder), Congo red (gellingagent) and Igapal (a commercially available Wetting agent) were added tothe mixture. In this particular example, 30 cc. of the P.V.A. 71-24, 0.3gram of Congo red and 0.3 cc. of Igapal were used in a batch consistingof about 300 grams of ferrite in 110 ml. of water. Other water-solublebinders, such as gelatin, can be used with or in lieu of the P.V.A.;other watersoluble gelling agents (e.g., ammonium pentaborate) can beused with or in lieu of the Congo red. Similarly, the Marasperse and/orlgapal can be replaced wholly or partly by other suitable dispersing andwetting agents, respectively.

This mixture is a relatively thick gel at this point and is put in acontainer and heated to a temperature of about 55 C. After heating itconverts to a somewhat viscous liquid. The heated liquid mixture then isvigorously agitated to entrain bubbles of air or other ambient gaseousmedium. The aeration may be accomplished conveniently by whipping themixture with a conventional motor-driven food mixer, such as a SunbeamMixmaster. Sufficient aeration usually requires whipping for eightminutes or more. Use of this method of agitation gives satisfactoryresults with the entrained gas bubbles dispersed more or less uniformlythroughout the mixture. It is pointed out that, in most cases, thewhipping would be carried out in an ordinary atmosphere; however, thiscould be done in an enclosure filled with some other gas and it is to beunderstood that the terms aerated, air bubbles, and the like are usedloosely throughout this description and the appended claims and areintended to encompass gases other than air.

The density of the finished ceramic ferrite elements produced by theprocess is determined by the ferrite used, the amount of water in themixture, the amount of wetting agent therein, and its temperature duringthe beating operation.

After being aerated in the manner just described,-the

foamed dispersion is poured into paper-lined, open mesh Wire baskets,where it is cooled down to room temperature so that it solidifies. Thenit is dried thoroughly, which may take from one to three days at roomtemperature and ordinary atmospheric conditions. After having beendried, the green ceramic elements have interstices or crevices formed byair bubbles throughout. The nickel ferrite elements had a bulk densityof the order of onefourth of the theoretical (or microscopic) density ofnickel ferrite (about 5.35 grams/cc.) or the maximum density obtainableas a practical matter in compacted, fired nickel ferrite ceramic whichis better than 90% of the theoretical.

The paper is removed from the dried elements and the elements fired tomaturity in substantially the standard manner of the conventional denseceramic elements. In this particular example, the elements were fired atabout 1300" C. for one hour. The fired elements had a bulk density ofabout 1.16 whereas dry-pressed (solid) disks of the same material had adensity of about 5.

The magnetostrictive transducer element produced by 4 the foregoingprocess is of cellular structure throughout formed with separatemacroscopic interstices or crevices filled with air (or other gaseousmedium). Because of its cellulated, sponge-like construction thetransducer element has a bulk density which is much lower than thedensity of nickel ferrite. Depending upon the amount of water addedbefore stirring and the temperature during stirring, ceramic nickelferrite sponge may have a density Within the range from about 0.5 to3.0, with 1.1 being a typical value. Because of the lower density ofsponge ceramic materials, and because of the higher compliance of theelements due to their cellulated structure, the mechanical (acoustic)characteristic impedance of the sponge ceramic elements is much lowerthan for dense ceramic elements, this mechanical impedance being equalto /density/compliance. As a result, the sponge elements have a goodimpedance match with water, which makes them very good transducerelements for underwater operation.

The cellulated transducer elements of the present invention have beenfound to operate effectively over a much wider frequency band widtharound resonance than has been possible with transducers employing denseceramic elements. Consequently, the transducer elements of the presentinvention are capable of a rapid response to signals which start and endabruptly. Thus, transducers incorporating such elements are particularlywell adapted for echo-ranging using pulse techniques, and otherapplications where a short time constant is vital.

In addition, the cellular ceramic material of the present invention hasbeen found to be considerably easier to machine into a transducerelement of the desired configuration, such as by cutting with a hack-sawor sanding, than is the dense piezomagnetic ceramic.

It will be apparent to those skilled in the art to which this inventionpertains that the present transducer element has a reduced elastic crosscoupling, as a consequence of its cellular structure. For this reason,when the porous ceramic is operated in the parallel mode, radiatingacoustic energy from only one electroded face, there is relativelylittle energy radiated from the element transverse to this direction.Accordingly, the radiated acoustic energy is highly directive and byproper design a substantially single lobe pattern may be obtained, whichis particularly desirable in certain underwater applications. In thepast, because of the relatively high elastic cross coupling in denseceramic elements, it was not possible to operate such elements in theparallel mode where directivity was an important consideration, exceptby providing a number of elements of that type each elongated in theparallel mode direction and each having a relatively small radiatingface area and arranged in mosaic arrays which were difficult andexpensive to construct for operation in the desired manner. With thecellular ceramic element of the present invention, by proper design gooddirectivity may be obtained with an electroacoustic transducer employinga single ceramic element having a relatively large radiating face areawhich radiates energy in the 33, or parallel mode. A further importantconsideration worth noting is that any ceramic element used forelectromechanical transducer purposes has the optimum coupling whenoperated in its 33 mode; that is, for a given electrical energy inputmaximum conversion to mechanical energy is obtained by operating in thismode. Thus, in the present invention, a ceramic transducer element ofsimple and inexpensive configuration may be operated in its mosteffective mode (the 33 mode), without resulting in lack of directivityor substantial interference between the parallel and lateral modes.

FIGURE 2 illustrates a cellular ceramic element 10 according to thepresent invention produced by the method described in detail above andoutlined in FIG. 1. This element is here shown as toroidal inconfiguration. As indicated in the drawing, the element is of cellularconstruction, having separated macroscopic air holes or intersticesthroughout. This particular element was cut from a fired block of spongeferrite but it will be appreciated that the aerated mixture may be castdirectly into the desired shapes.

FIGURE 3 illustrates this cellular ceramic torus wrapped with a biasing(or polarizing) coil 12 and a signal coil 14. In operation, biasing coil12 is connected to a suitable source of D.-C. potential (not shown)which polarizesthe element magnetically while a signal applied to coil14 causesthe'element to vibrate in the radial mode at the signalfrequency. If desired, once polarized, the biasing voltage may be cutoff and the element operated on the remanent polarization.Alternatively, a polarizing permanent magnet can be used in place ofcoil 12 as hereinafter described in another embodiment.

In FIGURE 4, there is shown an underwater transducer employing atransducer element generally similar to that shown in FIG. 3. Theceramic element 10 is similar in all respects to that of FIG. 3, exceptthat its axial dimension is greater thus enabling it to function as acavity resonator. Element 10 is mounted on a sponge rubber pad 16 whichis full of air holes which act effectively to decouple the adjacent faceof the ceramic element. The mounting pad 16 is mounted on an open endedhousing base 18 across whose open end there extends a rubber cap 20. Theinterior of the housing is filled with oil. The lead-in conductors 22and 24 for the coils 12 and 14, respectively, on the ceramic elementextend into the housing through a fluid-tight seal 26.

In the operation of the transducer for transmitting acoustic energy aD.-C. biasing voltage is connected across conductors 22 and a voltage ofa predetermined frequency is applied across the signal conductors 24causing acoustic energy to be radiated from the ceramic element. Thisacoustic energy is transmitted through the oil and the rubber cap intothe surrounding water with very little energy loss therein since boththe oil and rubber have a very good impedance match with water.

Conversely, ifthe transducer is operated as a receiver, then acousticenergy transmitted through the water passes through the rubber cap 20and the oil in the housing and stresses the ceramic element 10, causingthe latter to produce a voltage across the conductor 24 which isrepresentative of the acoustic signal received.

FIGURE 5 illustrates, somewhat schematically, another magnetostrictiveunderwater transducer embodying the present invention. The transducer issimilar to that shown in FIGURE 4, comprising an oil-filled housing 18,having one side closed by an acoustically transparent rubber cap 20'.Mounted on a sponge pad 16' is a magnetostrictive transducer elementmade up of a pair of spaced, parallel sponge ferrite blocks 28, 30 and atransverse member 32 cemented to and connecting the adjacent ends of theblocks so as to form three sides of a rectangle. Preferably member 32 isa sponge ceramic material also albeit the material may beelectromechanically inert. Alternatively, the members 28, 30 and 32 maybe cast of sponge ferrite as an integral piece. Between the ends ofblocks 28, 30 remote from member 32 is a permanent magnet 34 whichpolarizes the ferrite blocks and completes the magnetic circuit.

Each of the ferrite blocks 28, 30 is wound with a respective signal coil36, 38. The signal coils are connected in series-aiding relation and areprovided with suitable leads which pass through a watertight seal 26' tothe exterior of housing 18' for connection to the signal source.

In operation the signal field developed by coils 36 and 38 causescorresponding longitudinal vibration of blocks 28 and 30 which drivetransverse member 32. Member 32 radiates acoustic energy through thecoupling fluid in the housing and rubber cap 20' with the surroundingwater or other transmission medium. As already explained in conjunctionwith FIGURE 4, the transducer also may be operated as a receiver inwhich case acoustic energy transmitted through the water passes throughthe rubber cap 20' and the coupling fluid in the housing and stressesblocks 28 and 30, thus developing a voltage across coils 36, 38 which isrepresentative of the acoustic signal received.

In the foregoing description, the material of which the transducerelement is composed has been specified as being nickel ferrite. However,it is to be understood that within the purview of the present invention,there may be employed other ferromagnetic ceramic materials which, whenpolarized, have a substantial electromechanical response, particularly apiezomagnetic response. By the term polarized as used herein is meanteither permanently polarized (i.e., having a remanent polarization) orelse subjected to a temporary polarizing field the time it is operatedso as to render it capable of an electromechanical response,particularly a piezomagnetic response. As examples of other suitableceramic ferrites, the ceramic may consist of a cobalt ferrite or amixture of nickel ferrite and cobalt or zinc ferrites.

A discussion of the piezomagnetic behavior of dense ferrite is given inan article by C. M. Van der Burgt entitled Ceramic Ferrite Resonatorspublished in the Journal of the Acoustic Society of America, November1956.

Insofar as the transducer element itself is concerned, without departingfrom the purview of this invention it may be made by processes otherthan that described herein, so long as it has the low density, cellularstructure which renders it capable of accomplishing the purposes of thisinvention.

Therefore, while there have been disclosed in the foregoing descriptiona specific presently preferred manner of practicing the process of thepresent invention and a specific preferred embodiment of the ceramictransducer element itself, it is to be understood that variousmodifications, omissions and refinements which depart from the disclosedembodiments of the process and product of the present invention may beadopted without departing from the spirit and scope of this invention.

I claim: I

1. An electromechanical transducer element comprising a body of cellularstructure formed with a multiplicity of macroscopic intersticesthroughout, each of said interstices being substantially smaller in anydirection than the dimension of said body in the same direction, saidbody consisting essentially of ferromagnetic ceramic material capable ofa substantial electromechanical response.

2. An electromechanical transducer element in the form of a fired bodyconsisting of polycrystalline ferromagnetic ceramic capable of asubstantial magnetostrictive response, the body having throughout itsextent a multiplicity of macroscopic interstices each of which issubstantially smaller in any direction than the dimension of the body inthe same direction, the body having a substantially lower bulk densitythan the theoretical density of said material.

3. An electromechanical transducer element comprising a body ofpolarizable ferromagnetic material of macroscopically cellular structurehaving a multiplicity of interstices throughout, each of saidinterstices being substantially smaller in any direction than thedimension of the body in the same direction.

4. An electromechanical transducer element according to claim 3, whereinsaid material is composed primarily of a ferromagnetic ferrite.

5. An electromechanical transducer element according to claim 3, whereinsaid material is composed primarily of at least one ferrite selectedfrom the group consisting of nickel ferrite, cobalt ferrite, mixednickel-cobalt ferrites, and mixed nickel-zinc ferrites.

6. An electromechanical transducer comprising an aerated body formedwith a multiplicity of macroscopic air holes throughout and consistingprimarily of polycrystalline ferromagnetic ceramic material ofcellulated structure throughout, each of said air holes beingsubstantially smaller in any direction than the dimension of the body inthe same direction, the bulk density of said body being substantiallylower than the theoretical micro scopic density of said ceramicmaterial; means for magg 2,723,239 Harvey Nov. 8, 1955 2,770,523 TooleNov. 13, 1956 2,904,395 Downs Sept. 15, 1959 netically polarizing saidbody; and means for supplying 5 an electromagnetic signal field to saidbody.

References Cited in the file of this patent UNITED STATES PATENTS Katoet a1. Oct. '9, Thuras Sept. 5, Schoenberg Aug. 21, Firth Sept. .4,Crowley Nov. 13,

OTHER REFERENCES Snoek: Physical III, No. 6, pp. 463-468, 476-479, 481,482, June 1936.

Harvey et al.: RCA Review, September 1950, pp. 10 344-349.

6. AN ELECTROMECHANICAL TRANSDUCER COMPRISING AN AERATED BODY FORMEDWITH A MULTIPLICITY OF MACROSCOPIC AIR HOLES THROUGHOUT AND CONSISTINGPRIMARILY OF POLYCRYSTLLINE FERROMAGNETIC CERAMIC MATERIAL OF CELLULATEDSTRUCTURE THROUGHOUT, EACH OF SAID AIR HOLES BEING SUBSTANTIALLY SMALLERIN ANY DIRECTION THAN THE DIMENSION OF THE BODY IN THE SAME DIRECTION,THE BULK DENSITY OF SAID BODY BEING SUBSTANTIALLY LOWER THAN THETHEORETICAL MICROSCOPIC DENSITY OF SAID CERAMIC MATERIAL; MEANS FORMAGNETICALLY POLARIZING SAID BODY; AND MEANS FOR SUPPLYING ANELECTROMAGNETIC SIGNAL FIELD TO SAID BODY.